Constant power thermal sensor

Abstract

A constant power thermal property sensor compensates for changes in the temperature of the medium into which the sensor is disposed. The sensor has two side-by-side probes mounted in the medium with resistances that vary with temperature. A constant power source produces a current through the first probe and a current source applies a current to the second probe at a fixed ratio of the current in the first probe. The voltages across the probes are monitored and compared, and a signal is produced which is divided by current through the first probe. The resulting signal is representative of the heat transfer between the probes and the medium. The signal does not appreciably vary with changes in the temperature of the medium, and the medium may be a liquid, a gas, a liquid flow, a gas flow, or a combination of media. In this construction, two principal applications for the sensor are liquid level detectors and flow monitors.

What is claimed is: 
     
       1. A sensor for determining thermal properties of a medium surrounding the sensor while compensating for changes in temperature of the medium, the sensor comprising: a first probe for being mounted in the medium, said first probe having a resistance which varies with temperature;   a second probe for being mounted in the medium, said second probe having a resistance which varies with temperature;   means for mounting said first and second probes side-by-side in the medium;   a constant power source connected for producing a current through and a voltage across at said first probe;   a current source connected for applying a current through said second probe at a fixed multiple of the current in said first probe sufficient to cause self-heating of said second probe, and producing a voltage across said second probe;   means connected for monitoring the voltages across said first and second probes; and   means for comparing the voltage across said first and second probes to produce a signal representative of the thermal properties of the medium.   
     
     
       2. The sensor of claim 1, wherein said means for comparing the voltages across said first and second probes to produce a signal representative of the thermal conductivity of the medium comprises: means for multiplying the voltage across the first probe by said fixed multiple;   means for producing a reference voltage signal that is proportional to the voltage across the first probe;   means for comparing the voltage across the second probe and the reference voltage signal to produce a difference voltage signal; and   means for dividing the difference voltage signal by a signal representative of the current through said second probe to produce a signal that is representative of the thermal properties of the medium.   
     
     
       3. The sensor of claim 1, wherein: the medium is a combination of liquid and gas, the liquid having a level to be measured; and   the signal representative of the thermal properties of the medium is a signal representative of the level of the liquid.   
     
     
       4. The sensor of claim 1, further comprising means for producing a signal representative of the temperature of the medium. 
     
     
       5. The sensor of claim 4, wherein said means for producing a signal representative of the ambient temperature of the medium comprises: means for multiplying the voltage across the first probe by said fixed multiple to produce a voltage signal that is proportional to the voltage across the first probe; and   means for dividing the voltage signal that is proportional to the voltage across the first probe by a signal representative of the current through the second probe to produce a signal that is representative of the ambient temperature of the medium.   
     
     
       6. A sensor for determining thermal properties of a medium surrounding the sensor while compensating for changes in temperature of the medium, the sensor comprising: a first probe for being mounted in the medium, said first probe having a resistance given by the equation:   R.sub.1 =R.sub.0 (1+αΔt.sub.a)        where R 1  equals said resistance of said first probe at an ambient temperature;   R 0  equals a resistance of said first probe at an initial temperature;   α equals a temperature coefficient of resistance of said first probe; and   Δt a  equals a difference between said initial temperature and said ambient temperature;     a second probe for being mounted in a medium, said second probe having a resistance given by the equation:   R.sub.2 =R.sub.0 (1+αΔt.sub.a +αΔt.sub.s)        where R 2  equals said resistance of said second probe at a self-heated temperature; and   Δt s  equals a difference between said ambient temperature and said self-heated temperature;     means for mounting said first and second probes side-by-side in the medium;   a constant power source connected for producing a current through and a voltage across said first probe, said voltage given by the equation:   V.sub.1 =I.sub.1 R.sub.1        where V 1  equals the voltage across said first probe; and   I 1  equals the current through said first probe;     a current source connected for applying a current through said second probe at a fixed multiple of said current through said first probe sufficient to cause self-heating of said second probe, said current through said second probe given by the equation:   I.sub.2 =kI.sub.1, and        where   2  equals said current through said second probe; and   k equals said fixed multiple, and said current source producing a voltage across said second probe, said voltage given by the equation:   V.sub.2 =I.sub.2 R.sub.2 =kI.sub.1 R.sub.2        where V 2  equals the voltage across said second probe;     means for multiplying said voltage across said first probe by said fixed multiple producing a multiplied voltage given by the equation:   Vm=kV.sub.1        where Vm equals said multiplied voltage;   means for subtracting said multiplied voltage from voltage across said second probe producing a resulting voltage given by the equation:   V.sub.T =V.sub.2 -kV.sub.1 =kI.sub.1 R.sub.0 αΔt.sub.s        where V T  equals said resulting voltage; and   means for dividing said resulting voltage by said current of said first probe, said means producing a signal given by the equation:   V.sub.T /I.sub.2 =V.sub.T /kI.sub.1 =R.sub.0 αΔt.sub.s,        whereby said signal is representative of the thermal properties of the medium.   
     
     
       7. The sensor of claim 6, further comprising: means for multiplying said voltage across said first probe by said fixed multiple and producing a multiplied voltage signal that is proportional to the voltage across said first probe, said multiplied voltage given by the equation:   V.sub.m =kV.sub.1        where V m  equals said multiplied voltage; and   means for dividing said multiplied voltage signal by said current through said second probe and producing a signal given by the equation:   V.sub.m /I.sub.2 -kV.sub.1 /kI.sub.1 =R.sub.0 αΔt.sub.a1,        whereby said signal is representative of said ambient temperature of the medium.   
     
     
       8. A sensor for determining thermal properties of a medium surrounding the sensor while compensating for changes in temperature of the medium, the sensor comprising: a first probe for being mounted in the medium, said first probe, having a resistance which varies with temperature;   a second probe for being mounted in the medium, said second probe having a resistance which varies with temperature;   means for mounting said first and second probes side-by-side in the medium;   a constant power source connected for producing a constant power;   a first current source connected to said constant power source for producing a current through and a voltage across said first probe;   a second current source connected for applying a current through said second probe at a fixed multiple of the current in said first probe and producing a voltage across said second probe;   a multiplier circuit for receiving the voltage across said first probe and multiplying said voltage by said fixed multiple producing a multiplied voltage signal;   a subtraction circuit for subtracting said multiplied voltage signal from the voltage across said second probe producing a resulting voltage; and   a divider circuit for dividing said resulting voltage by the current through said second probe and producing a signal that is representative of the thermal properties of the medium.   
     
     
       9. The sensor of claim 8, further comprising a divider circuit for dividing said multiplied voltage signal by the current through said second probe and producing a signal that is representative of the temperature of the medium. 
     
     
       10. A sensor for determining thermal properties of a medium surrounding the sensor while compensating for changes in temperature of the medium, the sensor comprising: first and second probes each having a resistance which varies with temperature;   means for mounting said probes side-by-side in the medium;   a constant power source connected for producing a current through and a voltage across each of said probes while maintaining constant power in at least one of said probes, and producing more current through said second probe than through said first probe sufficient to cause self-heating of said second probe, while maintaining a fixed ratio between the respective currents through said probes;   means for monitoring the voltages across said probes; and   means for comparing the voltages across said probes to produce a signal representative of the thermal properties of the medium.